The invention relates to a fuel injection apparatus of a type disclosed in German Pat. No. 958,440, corresponding to U.S. Pat. No. 2,831,433, in which the injection pump is embodied as a pump/nozzle assembly. The drive apparatus of this pump/nozzle assembly substantially comprises a rocker arm driven by a drive cam mounted on a cam shaft. The arm drives the pump piston through a push rod. The rocker arm is supported on an eccentric which is rotatable by an rpm-dependent control apparatus for the purpose of varying the onset of the fuel injection. The center of the eccentric is displaced substantially transversely to the longitudinal axis of the rocker arm. The pump piston has a first oblique control edge located on its front face which serves the purpose of varying the onset of the fuel injection and a second oblique control edge, by means of which the fuel quantity to be injected can be set in a known manner when the pump piston rotates in response to a setting device.
A further fuel injection apparatus of the same general type is known from German Offenlegungsschrift (laid open patent application) No. 2,558,699, corresponding to U.S. Pat. No. 4,092,964. The fuel injection apparatus disclosed therein includes an injection pump embodied as a pump/nozzle assembly having a two part pump piston, namely, a supply piston and a diverting piston. The two pistons enclose between them an equalizing chamber filled with fluid and serving as a hydraulic push rod. The equalizing chamber is blocked by an oblique control edge on the supply piston at the beginning of supply, remains closed during the supply stroke, and is opened by the diverting piston in order to terminate the supply stroke. By means of an electrically controlled setting device, the supply piston having the oblique control edge can be rotated, in order to initiate the pumping stroke of the diverting piston at a sooner or later point in time, by closing the equalizing chamber. During the supply stroke, the diverting piston supplies fuel from a pump working chamber to the injection nozzle, the pump working chamber being provided with a fuel quantity apportioned by a fuel apportionment apparatus.
These known fuel injection apparatuses, and all injection pumps driven mechanically by a drive cam, have the inherent disadvantage that the injection pressure which is proportional to the stroke speed of the pump piston, and therefore the peak pressure as well, varies in accordance with the engine speed; while, for example, in hydraulically driven pump/nozzle assemblies the stroke speed of the pump piston and thus the injection pressure corresponding to the servo pressure remains constant or is variable when the servo pressure is regulated. Fuel injection apparatuses with hydraulically driven pump/nozzle assemblies entail a very great expense because of the pressure unit for generating the servo pressure which is required in addition to the fuel apportionment apparatus.
In order to obtain optimal performance over the entire rpm range and in order to meet the ever more stringent conditions pertaining to air purity, it would be desirable to have a fuel injection apparatus which can be produced at favorable cost and in which the injection pressure or peak pressure can be controlled in accordance with engine speed.